I ran into an instance today wherein I needed to validate that a boolean field was either true or false and not null. I tried using validates :fieldname, :presence => true, but since :presence uses #blank? under the hood, it was reading false as not being present. (Why is false considered blank?)

Anyway, I needed a validator to test whether an attribute was either true or false and I couldn’t find anything among the standard validators, so I wrote my own.

Just plop this file in your app’s lib/validators directory.

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class TruthinessValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless value == true || value == false
      record.errors[attribute] << "must be true or false"
    end
  end
end

In your model, add the validation like so:

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class SomeModel < ActiveRecord::Base
  ...

  validates :field_name, :truthiness => true

  ...
end

For more information on writing validators, see Getting Started with Custom Rails3 Validators.